virtualization (3)

© 2010 IBM Corporation

Virtualization Technologies

Alex Landau ([email protected])IBM Haifa Research Lab

12 January 2010

© 2010 IBM Corporation2

What is virtualization?

Virtualization is way to run multiple operating systems and user applications on the same hardware

– E.g., run both Windows and Linux on the same laptop

How is it different from dual-boot?– Both OSes run simultaneously

The OSes are completely isolated from each other


Uses of virtualization

Server consolidation– Run a web server and a mail server on the same physical server

Easier development– Develop critical operating system components (file system, disk driver) without

affecting computer stability QA

– Testing a network product (e.g., a firewall) may require tens of computers– Try testing thoroughly a product at each pre-release milestone… and have a straight

face when your boss shows you the electricity bill Cloud computing

– The modern buzz-word– Amazon sells computing power– You pay for e.g., 2 CPU cores for 3 hours plus 10GB of network traffic


What’s new in that? We’ve been doing it for decades!

Indeed – an OS provides isolation between processes– Each has it’s own virtual memory– Controlled access to I/O devices (disk, network) via system calls– Process scheduler to decide which process runs on which CPU core

So what’s the hype about?

Try running Microsoft Exchange requiring Windows and your internal warehouse mgmt. application requiring Linux simultaneously on the same server!

Or better yet, try to persuade competing companies to run their processes side-by-side in Amazon’s cloud (had it not been virtualized)

Psychological effect – what sounds better?– You’re given your own virtual machine and you’re root there – do whatever you want– You can run certain processes, but you don’t get root, call our helpdesk with your

configuration requests and we’ll get back to you in 5 business days…


Two types of hypervisors

Definitions– Hypervisor (or VMM – Virtual Machine Monitor) is a software layer that allows several

virtual machines to run on a physical machine– The physical OS and hardware are called the Host– The virtual machine OS and applications are called the Guest

VMware ESX, Microsoft Hyper-V, Xen

Hardware

Hypervisor

VM1 VM2

Type 1 (bare-metal)

Host

Guest

Hardware

OS

Process Hypervisor

VM1 VM2

Type 2 (hosted)

VMware Workstation, Microsoft Virtual PC, Sun VirtualBox, QEMU, KVM

Host

Guest


Bare-metal or hosted?

Bare-metal– Has complete control over hardware– Doesn’t have to “fight” an OS

Hosted– Avoid code duplication: need not code a process scheduler, memory management

system – the OS already does that– Can run native processes alongside VMs– Familiar environment – how much CPU and memory does a VM take? Use top! How

big is the virtual disk? ls –l– Easy management – stop a VM? Sure, just kill it!

A combination– Mostly hosted, but some parts are inside the OS kernel for performance reasons– E.g., KVM


How to run a VM? Emulate!

Do whatever the CPU does but in software Fetch the next instruction

Decode – is it an ADD, a XOR, a MOV?

Execute – using the emulated registers and memory

Example:

addl %ebx, %eax

is emulated as:

enum {EAX=0, EBX=1, ECX=2, EDX=3, …};

unsigned long regs[8];

regs[EAX] += regs[EBX];


How to run a VM? Emulate!

Pro:– Simple!

Con:– Slooooooooow

Example hypervisor: BOCHS


How to run a VM? Trap and emulate!

Run the VM directly on the CPU – no emulation!

Most of the code can execute just fine– E.g., addl %ebx, %eax

Some code needs hypervisor intervention– int $0x80– movl something, %cr3– I/O

Trap and emulate it!– E.g., if guest runs int $0x80, trap it and execute guest’s interrupt 0x80 handler


How to run a VM? Trap and emulate!

Pro:– Performance!

Cons:– Harder to implement– Need hardware support

• Not all “sensitive” instructions cause a trap when executed in usermode• E.g., POPF, that may be used to clear IF• This instruction does not trap, but value of IF does not change!

– This hardware support is called VMX (Intel) or SVM (AMD)– Exists in modern CPUs

Example hypervisor: KVM


How to run a VM? Dynamic (binary) translation!

Take a block of binary VM code that is about to be executed

Translate it on the fly to “safe” code (like JIT – just in time compilation)

Execute the new “safe” code directly on the CPU

Translation rules?– Most code translates identically (e.g., movl %eax, %ebx translates to itself)– “Sensitive” operations are translated into hypercalls

• Hypercall – call into the hypervisor to ask for service• Implemented as trapping instructions (unlike POPF)• Similar to syscall – call into the OS to request service


How to run a VM? Dynamic (binary) translation!

Pros:– No hardware support required– Performance – better than emulation

Cons:– Performance – worse than trap and emulate– Hard to implement – hypervisor needs on-the-fly x86-to-x86 binary compiler

Example hypervisors: VMware, QEMU


How to run a VM? Paravirtualization!

Does not run unmodified guest OSes

Requires guest OS to “know” it is running on top of a hypervisor

E.g., instead of doing cli to turn off interrupts, guest OS should do hypercall(DISABLE_INTERRUPTS)


How to run a VM? Paravirtualization!

Pros:– No hardware support required– Performance – better than emulation

Con:– Requires specifically modified guest– Same guest OS cannot run in the VM and bare-metal

Example hypervisor: Xen


Industry trends

Trap and emulate

With hardware support

VMX, SVM


I/O Virtualization

We saw methods to virtualize the CPU A computer is more than a CPU

Also need I/O!

Types of I/O:– Block (e.g., hard disk)– Network– Input (e.g., keyboard, mouse)– Sound– Video

Most performance critical (for servers):– Network– Block


Side note – How does a NIC (network interface card) driver work?

Transmit path:– OS prepares packet to transmit in a buffer in memory– Driver writes start address of buffer to register X of the NIC– Driver writes length of buffer to register Y– Driver writes ‘1’ (GO!) into register T– NIC reads packet from memory addresses [X,X+Y) and sends it on the wire– NIC sends interrupt to host (TX complete, next packet please)

Receive path:– Driver prepares buffer to receive packet into– Driver writes start address of buffer to register X– Driver writes length of buffer to register Y– Driver writes ‘1’ (READY-TO-RECEIVE) into register R– When packet arrives, NIC copies it into memory at [X,X+Y)– NIC interrupts host (RX)– OS processes packet (e.g., wake the waiting process up)


I/O Virtualization? Emulate!

Hypervisor implements virtual NIC (by the specification of a real NIC, e.g., Intel, Realtek, Broadcom)

NIC registers (X, Y, Z, T, R, …) are just variables in hypervisor (host) memory If guest writes ‘1’ to register T, hypervisor reads buffer from memory [X,X+Y) and passes

it to physical NIC driver for transmission

When physical NIC interrupts (TX complete), hypervisor injects TX complete interrupt into guest

Similar for RX path


I/O Virtualization? Emulate!

Pro:– Unmodified guest (guest already has drivers for Intel NICs…)

Cons:– Slow – every access to every NIC register causes a VM exit (trap to hypervisor)– Hypervisor needs to emulate complex hardware

Example hypervisors: QEMU, KVM, VMware (without VMware Tools)


I/O Virtualization? Paravirtualize!

Add virtual NIC driver into guest (frontend)

Implement the virtual NIC in the hypervisor (backend)

Everything works just like in the emulation case…

…except – protocol between frontend and backend

Protocol in emulation case: – Guest writes registers X, Y, waits at least 3 nano-sec and writes to register T– Hypervisor infers guest wants to transmit packet

Paravirtual protocol: – Guest does a hypercall, passes it start address and length as arguments– Hypervisor knows what it should do

Paravirtual protocol can be high-level, e.g., ring of buffers to transmit (so NIC doesn’t stay idle after one transmission), and independent of particular NIC registers


I/O Virtualization? Paravirtualize!

Pro:– Fast – no need to emulate physical device

Con:– Requires guest driver

Example hypervisors: QEMU, KVM, VMware (with VMware Tools), Xen

How is paravirtual I/O different from paravirtual guest?– Paravirtual guest requires to modify whole OS

• Try doing it on Windows (without source code), or even Linux (lots of changes)– Paravirtual I/O requires the addition of a single driver to a guest

• Easy to do on both Windows and Linux guests


I/O Virtualization? Direct access / direct assignment!

“Pull” NIC out of the host, and “plug” it into the guest

Guest is allowed to access NIC registers directly, no hypervisor intervention

Host can’t access NIC anymore


I/O Virtualization? Direct access / direct assignment!

Pro:– As fast as possible!

Cons:– Need NIC per guest– Plus one for host– Can’t do “cool stuff”

• Encapsulate guest packets, monitor, modify them at the hypervisor level

Example hypervisors: KVM, Xen, VMware


I/O Virtualization? Emerging standard – SR-IOV!

Single root I/O virtualization

Contains a physical function controlled by the host, used to create virtual functions Each virtual function is assigned to a guest (like in direct assignment) Each guest thinks it has full control of NIC, accesses registers directly

NIC does multiplexing/demultiplexing of traffic


I/O Virtualization? Emerging standard – SR-IOV!

Pros:– As fast as possible!– Need only one NIC (as opposed to direct assignment)

Cons:– Emerging standard

• Few hypervisors fully support it• Expensive!• Requires new hardware

– Can’t do “cool stuff”

Example hypervisors: KVM, Xen, VMware


Industry trends on I/O virtualization

SR-IOV is the fastest

Also, the most expensive

Paravirtual I/O is cheap

But much worse performance

Companies (Red Hat, IBM, …) are looking at paravirtual I/O, trying to optimize it

Winner still unknown


Advanced topics

Memory over-commit

Nested virtualization

Live migration


The end!

Questions?

Alex Landau

[email protected]

virtualization (3)

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